U.S. patent number 7,418,017 [Application Number 11/492,248] was granted by the patent office on 2008-08-26 for interferometer, in particular for determining and stabilizing the relative phase of short pulses.
This patent grant is currently assigned to Menlo Systems GmbH. Invention is credited to Ronald Holzwarth, Michael Mei.
United States Patent |
7,418,017 |
Holzwarth , et al. |
August 26, 2008 |
Interferometer, in particular for determining and stabilizing the
relative phase of short pulses
Abstract
A description is given of an optical structure (100), in
particular for determining and stabilizing the relative phase of
short pulses, which contains a broadening device (6) for broadening
the frequency spectrum of pulses of electromagnetic radiation, and
a frequency multiplier device (8) for multiplying at least one
frequency component of the pulses, wherein a focusing lens optic
(7) is arranged between the broadening device and the frequency
multiplier device, which focusing lens optic can be used to focus
the pulses into the frequency multiplier device (8). Uses of this
optical structure are also described.
Inventors: |
Holzwarth; Ronald (Munich,
DE), Mei; Michael (Munich, DE) |
Assignee: |
Menlo Systems GmbH
(Martinsried, DE)
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Family
ID: |
37650247 |
Appl.
No.: |
11/492,248 |
Filed: |
July 25, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070071060 A1 |
Mar 29, 2007 |
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Foreign Application Priority Data
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Jul 27, 2005 [DE] |
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10 2005 035 173 |
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Current U.S.
Class: |
372/29.023;
372/29.02; 372/30 |
Current CPC
Class: |
G01J
11/00 (20130101) |
Current International
Class: |
H01S
3/13 (20060101) |
Field of
Search: |
;372/22,29.02,29.023-30 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Kakehata, Masayuki, et al., "Single-shot measurement of
carrier-envelope phase changes by spectral interferometry," Optics
Letters, vol. 26, No. 18, pp. 1436-1438, Sep. 2001. cited by other
.
Udem, Th., et al., "Optical frequency metrology," Insight Review
Articles, Nature, vol. 416, pp. 233-237, Mar. 2002. cited by other
.
Baltu{hacek over (s)}ka, A., et al., "Controlling the
Carrier-Envelope Phase of Ultrashort Light Pulses with Optical
Parametric Amplifiers," Physical Review Letters, vol. 88, No. 13,
pp. 133901-1-133901-4, Apr. 2002. cited by other .
Baltu{hacek over (s)}ka, A., et al., "Attosecond control of
electronic processes by intense light fields," Letters to Nature,
Nature, vol. 421, pp. 611-615, Feb. 2003. cited by other.
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Primary Examiner: Harvey; Minsun
Assistant Examiner: Nguyen; Phillip
Attorney, Agent or Firm: Schnader Harrison Segal & Lewis
LLP
Claims
The invention claimed is:
1. Phase stabilization device for pulses of electromagnetic
radiation, which comprises: an optical structure containing a
broadening device for broadening a frequency spectrum of pulses of
electromagnetic radiation, a frequency multiplier device for
multiplying at least one frequency component of the broadened
pulses, a focusing lens optic, which is arranged between the
broadening device and the frequency multiplier device and which can
be used to focus the broadened pulses into the frequency multiplier
device, and a spectrometer detecting interference patterns in the
light output of the frequency multiplier device, and a device for
generating a control signal from the interference patterns.
2. Laser device, which comprises: a pulse source generating pulses
of electromagnetic radiation, and the phase stabilization device
according to claim 1, wherein the device for generating a control
signal forms part of a control loop which can be used to control
the pulse source.
3. Phase stabilization device according to claim 1, which is
adapted for determining and stabilizing the relative
carrier-envelop phase of short pulses.
4. Phase stabilization device according to claim 1, in which no
mirror optic is provided on the optical path between the broadening
device and the frequency multiplier device.
5. Optical method for carrier-envelope-phase stabilization of
pulses of electromagnetic radiation, comprising the steps:
broadening of a frequency spectrum of pulses of electromagnetic
radiation in a broadening device, and transferring the broadened
pulses of electromagnetic radiation by means of a focusing lens
optic from the broadening device to a frequency multiplier device,
detecting interference patterns in the light output of the
frequency multiplier device using a spectrometer, and generating a
control signal from the interference patterns.
6. Optical method according to claim 5, wherein the focusing lens
optic is arranged between the broadening device and the frequency
multiplier device, the step of transferring the pulses comprises
focusing the pulses into the frequency multiplier device using the
focusing lens optic, and the optical method comprises the further
step of multiplying at least one frequency component of the pulses
using the frequency multiplier device.
Description
This application is based on, and claims priority to, German patent
application, serial number 10 2005 035 173.5, having a filing date
of Jul. 27, 2005, and entitled Interferometer, insbesondere fur die
Bestimmung und Stabilisierung der relativen Phase kurzer Pulse.
SUBJECT OF THE INVENTION
The invention relates to an optical set-up for handling pulses of
electromagnetic radiation, and in particular to a compact
interferometer, for example for determining and stabilizing the
relative phase of short pulses. The invention also relates to
methods for handling pulses of electromagnetic radiation, in
particular laser pulses.
PRIOR ART
Mode-coupled short-pulse lasers emit a periodic pulse train. In
order to visualize the important processes which occur therein, the
idealized case of a short pulse which circulates in a laser
resonator with the length L and with the carrier frequency
.omega..sub.c will be considered first. This is shown in FIG. 1.
Each time the pulse occurs on the output coupler of the laser
resonator, a copy of the pulse is output. The output pulses are
separated from one another in time terms by the cycle time of the
pulses in the resonator T=v.sub.g/2L, wherein v.sub.g is the
average group velocity in the resonator and L is the length of the
(linear) resonator. However, the output pulses are not identical.
The envelope of a pulse moves at the group velocity v.sub.g,
whereas on the other hand the electric carrier field on which it is
based moves at its phase velocity. As a result, the phase between
the envelope and the electric field is shifted by .DELTA..phi. from
pulse to pulse, as shown in FIG. 1. The envelope itself is
periodic, i.e. A(t)=A(t-T), whereas the electric field on the other
hand is not. The electric field can be expressed accordingly as
E(t)=Re(A(t)exp(-i.omega..sub.ct))=Re(.SIGMA..sub.nA.sub.nexp(-i(.omega..-
sub.c+n.omega..sub.r)t)) (1)
Here, A.sub.n are the Fourier components of A(t). Under the
prerequisite of a periodic envelope, the resulting spectrum can
therefore be described as a comb of laser modes, separated by the
pulse repetition rate. Since .omega..sub.c is not necessarily a
multiple of .omega..sub.r, the modes are obviously shifted with
respect to the precise harmonic of the pulse repetition rate, and
the following applies: .omega..sub.n=n.omega..sub.r+.omega..sub.o
(2) with a large (.apprxeq.10.sup.6) even number n. This equation
shows two radio frequencies .omega..sub.r and .omega..sub.o on the
optical frequency .omega..sub.n. This can be used for optical
frequency metrology and is described for example in EP 1 161 782
and in "Nature", vol. 416, 2002, page 233.
For many practical applications, the offset frequency of the
frequency comb has to be stabilized. In order to detect the offset
frequency, use is made for example of a structure as shown in FIG.
2. If the spectrum covers an entire optical octave, it contains two
modes with the mode numbers n and 2n. If the mode with the mode
number n has its frequency doubled and is made to beat with the
mode 2n, then according to equation 2 the desired frequency is
obtained
.omega..sub.o=2(n.omega..sub.r+.omega..sub.o)-(2n.omega..sub.r+.omega..su-
b.o).
In practice, the high peak intensity can be used to double in a
very efficient manner a large number of modes in the vicinity of n,
in order then to make them beat with an equal number of modes in
the vicinity of 2n. If the propagation times of the pulses are
selected correctly, all the beat signals are constructively
superposed on one another and thus amplify the signal again. The
signal obtained in this way can then be stabilized to a predefined
frequency or to zero. If the offset frequency is stabilized to
zero, each pulse has an identical appearance. If the offset
frequency is stabilized to 1/4 of the repetition rate, each 4th
pulse is identical.
This in turn is important for high-grade non-linear processes.
Processes in which the electric field occurs at a high power, such
as for example the generation of high harmonics or "above threshold
ionization" react sensitively to whether the electric field has or
has not reached its maximum below the envelope, that is to say
whether the pulse in question is a sine or cosine pulse. This is
illustrated in FIG. 3. FIG. 3 shows the calculated intensity at 3.2
nm and the generated intensity in the case of a sine and a cosine
pulse. For the cosine pulse, the electric field reaches its maximum
below the envelope.
In order to be able to observe such processes, it is important that
all the pulses are identical, that is to say have the same phase
difference between the envelope and the electric field. Moreover,
for such effects, usually a very high pulse energy of a few .mu.J
to several mJ is necessary, and this requires further amplification
of the pulses. Such a high-power system is shown in FIG. 4.
The starting point of the system is a phase-stabilized 10 fs laser
system. This consists of a Ti:sapphire fs laser (Femtosource
Compact Pro, Femtolasers) which is mode-coupled via the Kerr effect
and uses special "chirped" mirrors for dispersion compensation, and
of a phase stabilization device (XPS 800 unit, MenloSystems GmbH).
This phase stabilization device uses an f:2f interferometer
(interferometer I in FIG. 4) and phase lock electronics. These
phase lock electronics in turn control the acousto-optical
modulator (AOM in FIG. 4) via a suitable driver. Around 50% of the
output power of the fs laser is coupled into a photonic crystal
fiber in order to generate a spectrum having a width of one octave.
Further details are described in the handbook for the XPS 800 phase
stabilization device. In the text which follows, the long-wave part
f.sub.low of the broadened spectrum has its frequency doubled in
order to be able to observe a beat signal with the short-wave part
of the broadened comb whereby f.sub.high=2f.sub.low. The optical
set-up for this will be referred to below as an f:f2
interferometer. The phase of the 2 interfering quasi-monochromatic
wave packets differs by 2.phi.-.phi.+.PHI., wherein .PHI. is an
unknown constant phase, which prevents it from being possible for
an absolute measurement to be carried out with such an arrangement
.phi.. Although the absolute position of the phase .phi. between
the envelope and the carrier wave is unknown, the change in the
information obtained here can be used to stabilize it. The output
pulses then all have the same phase position, even though said
phase position is unknown. The f:2f beat signal
f.sub.o=.DELTA..phi.f.sub.r/2.pi. is observed at around 530 nm and
then is fed to a digital phase detector which carries out a
comparison with a reference which is generated by dividing the
pulse repetition frequency by a factor of 4. The stabilization loop
forces the two signals, that is to say 1/4 f.sub.r and f.sub.o to
oscillate in phase. The following is thus obtained for the
pulse-to-pulse phase shift: .DELTA..phi.=1/4 2.pi., that is to say
that each 4th pulse is identical.
The coarse adjustment of .DELTA..phi. is achieved by adjusting the
optical path length by a quartz wedge within the laser resonator.
More or less glass is therefore introduced into the resonator.
Close to the desired value, that is to say at around 20 MHz in the
case of a repetition frequency of 80 MHz, the electronic control is
switched on. To this end, a fine adjustment of .DELTA..phi. is
carried out via the non-linear effects in the laser crystal. For
this purpose, the pump power of the fs laser is adjusted
accordingly by means of an acousto-optical intensity modulator.
As a result, each 4th pulse in the 80 MHz pulse train is identical.
If each 80,000th pulse is then selected via a pulse picker, each of
these pulses has the same phase position. These selected pulses are
then amplified in the multipass amplifier.
Unfortunately, the phase does not remain constant in the amplifier
but rather drifts due to instabilities. In order to be able to
stabilize the phase, which now changes more slowly, use is made of
a further f:2f interferometer ("interferometer II" in FIG. 4). Due
to the low pulse repetition rate and the fact that the phase
changes only slowly, the spectral interference is observed here and
is evaluated on a computer by means of Fourier transform
algorithms. Due to the high pulse power, in order to generate the
spectral interference it is sufficient here to focus the pulses
into a sapphire plate. The white light that is generated is
collimated by means of a curved mirror and again focused into a
crystal in order to double the frequency. The error signal
resulting from the evaluation of the spectral interference is added
to the initial offset of the PI control loop in the 1st (fast)
interferometer and can then adjust the phase, once again via the
AOM.
If, in the case of short pulses, the offset frequency is low, that
is to say for example is only a few Hertz or even mHz (as in the
above case based on an amplifier system), an interference pattern
with a certain modulation frequency can be observed on a
commercially available spectrometer. The interference bands then
run through the image at the offset frequency. If the offset
frequency is low enough and the spectrometer is fast enough, it is
thus possible to follow the movement of these interference
bands.
It is often desirable to make the offset frequency equal to zero.
In this case, the pulses are referred to as phase-stable pulses. To
this end, a control system is introduced which keeps the position
of the interference bands constant.
The apparatus described here is described in detail in "Nature",
vol. 421, 2003, page 614 ("Attosecond control of electronic
processes by intense light fields"). With regard to the properties
and technical function of this apparatus, this publication is
introduced into the present specification by reference. Spectral
interference known in the art is also described in M. Kakehata et
al., Opt. Lett. 26, 1436 (2001) and A. Baltuska et al., PRL 88,
133901 (2002).
One disadvantage of the conventional optical set-up is in
particular the fact that the white light is collimated by means of
curved mirrors. The mirrors require a high level of complexity in
terms of adjustment and give rise to an astigmatism error.
OBJECTIVE OF THE INVENTION
The objective of the invention is to provide an improved optical
structure, in particular an improved interferometer, by means of
which the disadvantages of the prior art can be overcome. The
objective of the invention is also to provide a correspondingly
improved optical method, in particular a method for superposing
frequency components of short pulses (interferometric
superposition).
SUMMARY OF THE INVENTION
This objective is solved by an optical set-up, in particular an
interferometer, which--unlike the conventional interferometer (as
described above)--exhibits direct imaging of a white light focus
into a frequency multiplier device (in particular a frequency
doubling crystal) by means of a lens optic.
In particular, the frequency doubling crystal may be a periodically
poled crystal, for example consisting of KTP.
The optical structure (one embodiment is shown in FIG. 5) has the
following advantages: Use is no longer made of curved mirrors, and
as a result astigmatism is avoided and losses are minimized. By
imaging the white light focus by means of a lens into the frequency
doubling crystal, a saving can be made with regard to further
optics and adjustment complexity and the device can be of very
compact design. The light is guided "in line", that is to say
continuously on the optical axis. As a result, complex holding of
the crystals and complicated adjustment are avoided. The device can
be set up by means of simple rails or an optical bench system. By
means of the thickness of the lens, the spatial modulation
frequency (that is to say the distance between the interference
bands) can be adjusted without changing any other system parameters
or introducing additional elements. By virtue of the periodically
poled crystal, it is possible to prevent "walk off" as occurs in a
volume crystal. This is particularly advantageous for an "in line"
arrangement of the elements.
The invention also relates to a phase stabilization device for
pulses, which is equipped with the optical set-up, to a laser
device which is equipped with the phase stabilization device, and
to an optical method for imaging pulses in an optical structure for
the interferometric superposition of pulse frequency
components.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred features and embodiments of the invention are described
in the following with reference to the attached drawings, which
show in:
FIG. 1: a graphical illustration of a circulation of a very short
pulse in a laser resonator with dispersion,
FIG. 2: a graphic illustration of a spectrum which covers an entire
optical octave,
FIG. 3: a graphic illustration of a calculated intensity at 3.2
nm,
FIG. 4: a structure of a conventional phase-stabilized system for
generating a high harmonic using an amplifier,
FIG. 5: an embodiment of an optical structure according to the
invention,
FIG. 6: interference patterns, recorded using the spectrometer
shown in FIG. 5.
PREFERRED EMBODIMENT
FIG. 1 illustrates a circulation of a very short pulse in a laser
resonator with dispersion. While the envelope moves at the group
velocity v.sub.g=d.omega./dk, the carrier phase runs at the phase
velocity v.sub.p=.omega./k, so that, after each cycle, the relative
phase between the carrier wave and the envelope increases by an
angle .DELTA..phi.. The spectrum shown in the lower part is
obtained through Fourier transformation of the strictly periodic
envelope. This spectrum consists of modes spaced apart by the pulse
repetition rate .omega..sub.r, which are shifted by
.omega..sub.o=.DELTA..phi./T from the harmonic of .omega..sub.r,
wherein T=2.pi./.omega..sub.r represents the pulse cycle time.
FIG. 2 shows that a spectrum which covers an entire optical octave
which contains two modes with the mode number n and 2n. If the mode
with the mode number n has its frequency doubled and is made to
beat with the mode 2n, then according to equation 2 the desired
frequency is obtained
.omega..sub.o=2(n.omega..sub.r+.omega..sub.o)-(2n.omega..sub.r+.omega..su-
b.o).
FIG. 3 illustrates the calculated intensity at 3.2 nm and the
generated intensity in the case of a sine and cosine pulse. For the
cosine pulse, the electric field reaches its maximum below the
envelope.
The mode of operation of the optical structure according to the
invention as shown in FIG. 5, in particular of the improved
interferometer, will be described below. Pulsed light from a pulse
source 200 (for example from a laser source and/or an amplifier
system) impinges on a beam splitter 1. Part of the light is output
for further use and part of the light, for example 1%, is fed into
the interferometer 100. Firstly, the intensity of the light can be
adjusted, namely by means of components, which are preferably
provided and which comprise an adjustable grey filter 3 and/or an
iris diaphragm 2,3 which can be closed to a greater or lesser
extent as necessary. The polarization of the impinging light can be
adjusted by means of a wave plate 4 (800 nm half-wave plate). The
light is then focused by an optic 5 (lens) into a crystal 6 in
order to generate white light. The crystal 6 serves as a broadening
device. The crystal 6 may consist for example of sapphire or quartz
glass and has a thickness of between 0.5 and 3 mm. In particular,
light which covers an optical octave (for example from around 500
nm to 1 .mu.m, white light) is generated here. This light is imaged
through a further optic 7 (lens) into a crystal 8 for the purpose
of frequency multiplication (in particular, generation of the 2nd
harmonic of the fundamental wave, SHG, second harmonic generation).
The optic 7 serves to recollimate and focus the white light. The
focal length of this imaging lens 7 is for example within the range
from 10 to 100 mm; it may also be an achromatic lens or a lens
system.
In the prior art, the process is carried out using curved mirrors
(see above). The reason for this can be seen in the management of
dispersion. Since the curved mirrors do not exhibit (if they are
coated with metal) or exhibit only very little of group velocity
dispersion, the frequency of the interference bands is not changed
at said mirrors. The advantage of the arrangement according to the
invention here, on the other hand, lies in the fact that the band
frequency can be adjusted via the lens thickness, thereby omitting
the problem concerning astigmatism during imaging and providing a
greatly simplified "in line" optical structure.
The SHG crystal 8 serves as a frequency multiplier device. It may
be either a conventional volume crystal or a periodically poled
crystal, for example consisting of lithium niobate or KTP. Its
length is in the range from 0.5 to 5 mm.
The output light is again collimated by a further lens 9 and is
coupled into a glass fiber by means of a suitable optic 12. Before
being coupled into an optical fiber 13 (for example a glass fiber),
an adjustable (rotatable) polarizer 10 is provided for setting the
correct polarization mix between the fundamental wave and the 2nd
harmonic (only necessary in the case of a volume crystal). A
bandpass filter 11 for green or blue light may advantageously be
provided in order to prevent saturation of the spectrometer 14.
The light coupled into the fiber 13 is finally analyzed in the
spectrometer 14. The typical interference bands are observed, as
shown in FIG. 6. The spectrometer is a commercially available
spectrometer comprising a grating and a CCD line camera with for
example approximately 2000 or 4000 pixels and a resolution of 0.1
to 5 nm.
The interference bands thus detected are then used to keep the
offset frequency constant. To this end, the band frequency is
evaluated by means of Fourier transformation and the associated
phase is calculated. An error signal for a PID (Proportional
Integral Differential) controller is in turn derived therefrom, as
known from control engineering.
Optionally, at least one wedge 15 (drawn with dashed line) can be
provided with the embodiment of FIG. 5. The at least one wedge 15
can be used for adjusting the phace by shifting the wedge.
Preferably, a double wedge (wedge pair) is provided in order to
avoid a beam shift by moving the wedge. Preferably, the at least
one wedge 15 is positioned after the laser-amplifier-combination
200.
An analogue voltage signal is generated as the control signal by
means of a device for generating a control signal (in particular a
digital/analogue converter), and this signal is added to the input
offset of the PI controller of the fast branch. Further details
concerning control of the pulse source on the basis of the control
signal will preferably be embodied as described above with
reference to FIG. 4 or in the operating instructions for the XPS
800 apparatus from MenloSystems GmbH, the contents of which are
hereby introduced by way of reference into the present description.
In this way, the phase between the envelope and the electric field
can be kept constant.
The features of the invention which are disclosed in the above
description, the drawings and the claims may be important both
individually and in combination with one another for implementing
the invention in its various embodiments.
* * * * *